If you want to launch the iPhone's copy of Safari with a particular URL, use the following code:
Code:
NSURL *theUrl = [NSURL URLWithString:url];
[[UIApplication sharedApplication] openURL:theUrl];
(Assumes "url" is an NSString containing the URL you want to launch.)
This will exit your app and fire up Safari with the URL you specify, assuming, of course, that the URL starts with http: or https: